One type of package used to house microelectronic devices, such as silicon based integrated circuits, is a molded plastic package. The package is low cost, easy to assemble and provides adequate protection of the device from water vapor and other contaminants. During assembly, a device is mounted on a die attach paddle and electrically interconnected to the lead frame. The die attach paddle, electronic device and inner portion of the lead frame are then encapsulated in a molding resin such as by transfer molding. The resin forms a hard, relatively moisture impervious shell to protect both the device and electrical connections.
After encapsulation, the outer leads of the lead frame may be soldered to a printed circuit board or other external device. The temperature of the package during soldering may rise to around 260.degree. C. Any water vapor trapped within the package will convert to steam and expand. One site for the accumulation of water is under the die attach paddle. When the accumulated water expands, the base of the molded plastic package deforms, leading to what is known as the "pop corn effect".
Moisture also penetrates the plastic package along the interface between the leads and molding resin. The adhesion between the resin and leads is primarily mechanical in nature and a gap may exist between the leads and encapsulation. Moisture travels through the gap, accumulating at the die attach paddle.
Various mechanisms to improve the bond between the lead frame and molding resin have been proposed. For example, U.S. Pat. No. 4,862,246 to Masuda et al discloses forming a series of hemispherical depressions in the die attach paddle to increase the adhesion of the paddle to the molding resin.
A layer of dull nickel applied to a lead frame has been found to enhance the epoxy bond as disclosed in U.S. Pat. No. 4,888,449 to Crane et al. The Crane et al patent also discloses coating metallic package components with a metal or metal alloy selected from the group consisting of nickel, tin, cobalt, chromium, molybdenum, tungsten, titanium, zirconium, hafnium, niobium, tantalum, germanium, aluminum and alloys of the foregoing metals. The Crane et al patent is incorporated in its entirety herein.
Another patent which discloses a metallic coating to improve the adhesion between a metallic lead frame and a polymer resin is U.S. Pat. No. 4,707,724 to Suzuki et al. The Suzuki et al patent discloses coating a die attach paddle with an alloy of tin/nickel or iron/nickel.
Non-metallic coatings have also been disclosed. For example, U.S. Pat. No. 4,428,987 to Bell et al discloses pretreating a copper surface to improve adhesion. The surface is electrolytically reduced and then coated with a benzotriazole solution.
U.S. Pat. No. 5,122,858 to Mahulikar et al discloses that coating a lead frame with a thin polymeric layer improves the adhesion between the lead frame and molding resin. U.S. Pat. No. 5,122,858 is incorporated in its entirety herein.
Molded plastic packages have a monolithic body. Other electronic packages are formed from discrete base and cover components. One such package is disclosed in U.S. Pat. No. 4,939,316 to Mahulikar et al. The package has anodized aluminum base and cover components with a lead frame disposed therebetween. A polymer adhesive bonds the lead frame to both the base and cover. Other packages having discrete bases and covers have copper, copper alloy or plastic components. Each of these packages could be improved if the adhesion between the metallic lead frame and the polymer resin is improved.